1. Field of the Invention
This invention relates to a brake disc and particularly to a brake disc for a motorcycle.
2. Description of Related Art
In vehicles, such as motorcycles, a disc brake has been widely in use as a control device. In such a disc brake device, a brake disc is mounted to a wheel hub of a front or rear wheel of the vehicle. A caliper having brake pads therein is provided on the body frame side of the brake disc at the outside circumferential portion thereof. The brake pads are pressed against the brake disc from both sides by hydraulic pistons, and friction created between the brake disc and the pads can brake rotation of the disc.
In such a disc brake device, since the temperature of the brake disc is raised by the frictional heat to cause thermal deformation, such as warping of the disc, it is necessary to dissipate frictional heat for improving a heat dissipation property so as to prevent deformation such as warping of the disc.
In conventional devices, numerous holes are provided in the brake disc to facilitate heat dissipation. To further improve the heat dissipation property for suppressing thermal deformation, one brake disc has been proposed in Japanese Patent Document JP-A-Hei08-210466. Referring to FIG. 8, the configuration of the brake disc disclosed in the foregoing patent document will be described.
Brake disc 110 shown in FIG. 8 is a modified disk of a configuration disclosed in FIG. 16 of JP-A-Hei08-210466. Brake disc 110 comprises a hub section 113 with a round and inside circumference, an annular braking section 114, and a plurality of arm sections 115 connecting the hub section 113 and the braking section 114.
More specifically, the brake disc 110 is formed integrally of a carbon steel plate, and comprises the hub section 113 having a fitting hole 112 for an axle to pass through, the annular braking section 114 on the outside circumferential side, and a plurality of arm sections 115 connecting the hub section 113 and braking section 114 circumferentially at a certain interval.
The braking section 114 is formed with a plurality of slots 116 at a certain interval, and a center Q1 of the head on the outside circumferential side of each connecting section 117 disposed between slits 116 is offset from a center Q2 of its base on the inside circumferential side by a fixed angle α in the circumferential direction. Between the braking section 114 and hub section 113 are formed alternately arm sections 115 and reduction holes 118 for reduction of weight. The hub section 113 is formed, near the bases of the arm sections 115, with mounting holes 111 for fixing the brake disc.
The plurality of arm sections 115 are provided such that centers P10 of their heads on the braking section 114 side and centers P11 of their bases on the hub section 113 side are offset in one circumferential direction by displacement angles θ10, θ11 and θ12 in this order repeatedly. Center lines L10 of the arm sections 115 are disposed oblique in one direction to radial line segments of the brake disc 10 crossing these center lines. Also, in every part of each arm section 115 along its entire length is provided a longitudinal line segment L12 crossing the arm section 115 radially of the brake disc 10 to prevent direct connection of the hub section 113 and braking section 114.
Now, referring to FIGS. 9(A) and (B), operation of the brake disc 110 is described. Since the centers P10 of the heads of the arm sections 115 and the centers P11 of their bases are offset in one circumferential direction by displacement angles θ10, θ11 and θ12 in this order repeatedly and the arm sections 115 are disposed oblique to the radial line segments of the brake disc 110, the expansion of the braking section 114 in the radial direction, as shown in FIG. 9(A) by phantom lines, is transformed into the elastic deformation of the arm sections 115, and the contraction of the braking section 114 in the radial direction, as shown in FIG. 9(B) by phantom lines, is transformed into the elastic deformation of the arm sections 115.
That is, in the case where heat generation during braking causes expansion of the braking section 114, even if radial tensile stresses F12 are exerted on the arm sections 115, as shown in FIG. 9(A), these tensile stresses F12 are absorbed by the arm sections 115 and reduction holes 118 being deformed as shown by the phantom lines. Also, even if radial compressive stresses F11 are exerted on the arm sections 115 as a result of contraction of the braking section 114 by the heat treatment applied to the braking section 114 during manufacturing of the brake disc 10, as shown in FIG. 9(B), these compressive stresses F11 are absorbed by the arm sections 115 and reduction holes 118 being deformed as shown by the phantom lines. Therefore, compressive stresses on the hub section 113 are decreased, deformation of the hub section 113 toward its center is prevented, and the sufficient roundness of the fitting hole 112 can be achieved without machining or the like after heat treatment.
In the brake disc disclosed in JP-A-Hei08-210466, thermal stress causing compressive force or tensile force exerted on the hub section are decreased without need of a complicated structure of the brake disc, and deformation of the inside circumferential surface of the hub section or warping of the whole disc can be effectively prevented for the transmission of braking force.
However, since in the brake disc of JP-A-Hei08-210466, at least an annular hub section is considered as an essential component while prevention of deformation of the inside circumferential surface of the hub section is pursued, it is difficult to expect further elastic deformation of the arm sections. That is, there is provided an arm section (connecting arm) extending from an arcuate portion connecting adjacent mounting holes in the annular hub section to a ring section, so that elastic deformation of the arm section is suppressed, which may provide a case where the brake disc as a hole is unable to absorb sufficiently thermal stress due to thermal expansion of the braking section.